Fluororesins are excellent in such characteristics as thermal resistance, chemical resistance, solvent resistance and insulating properties and, therefore, such products as tubes, covered electric wires, pipes and filaments can be obtained by melt-extrusion molding them. In particular, fluororesins comprising a tetrafluoroethylene [TFE]/hexafluoropropylene [HFP]-based copolymer [FEP] are low in permittivity and dielectric loss tangent and have good insulating properties and are suitably used in the field of application as coverings for electric wires such as cables and wires.
The FEP-containing resin compositions so far proposed as being suited for use in covering electric wires are, for example, substantially alkali metal salt-free ones having an HFPI of about 2.8 to 5.3, a melt flow rate [MFR] of 30±3 g/10 minutes and a number of unstable terminal groups of not larger than about 50 per 1×106 carbon atoms (cf. e.g. Patent Document 1: U.S. Pat. No. 7,126,056; Patent Document 2: United States Patent Application Publication 2004/0242819; Patent Document 3: United States Patent Application Publication 2006/0276604). However, the temperature range employable on the occasion of molding the electric wire is very narrow, and the molding stability may rapidly decrease under conditions outside this temperature range.
A FEP-containing fluororesin composition which has a sodium metal element content of 5 to 100 ppm and contains 0.01 to 3 parts by mass, per 100 parts by mass of the FEP, of a polytetrafluoroethylene [PTFE] having a specific standard specific gravity and which is obtainable by mixing an aqueous FEP dispersion with an aqueous PTFE dispersion, followed by coagulation has been proposed (cf. e.g. Patent Document 4: International Publication WO 2006/123694).